This invention relates to transport systems for digital scanning of the frames of a motion picture film. More specifically, the invention is directed to a method of, and the construction of apparatus for transporting film without slack or interference.
Prior art film transports have apparatus between the sprocket wheels which can interfere with even illumination of the film from behind, and with unobstructed sensing of the light transmitted through the film by a CCD.
Slack in the film is also caused when axially displaced sprocket wheels are used due to differences in rotational speed between the sprocket wheels. Pull-down claws, as are used in the prior art, also cause unevenness of movement and bunching of the film.
U.S. Pat. Nos. 5,088,813 and 5,548,328 to Wakefield disclose a scanner in which film is transported over two axially aligned spaced wheels and in which the film is illuminated from behind and the light transmitted through the film is focused by a lens onto a CCD. However, Wakefield""s wheels are not able to have the film transmit torque from a driven wheel to a free wheel. Instead, Wakefield uses two respective friction belts which engage respective surfaces of the two wheels, both belts being driven by a common shaft. The belts and shaft can be obstacles to optimum placement of the film illumination source and scanning sensor. Also, slippage between the belts and shaft can result in the application of a net torque which can deform the film and prevent attainment of proper positioning for distortion-free scans.
The aforementioned problems of the prior art are overcome by the present invention which provides a transport system for high speed digital scanning of the frames of a motion picture film, referred to as a xe2x80x9crotary gatexe2x80x9d. In a rotary gate for film scanning, according to the invention, two axially aligned film sprocket wheels are spaced apart in parallel planes with no connection between them. The bottom sprocket wheel is rotatably driven by a motor and engages the bottom row of sprocket holes of the film for transporting the film from a supply reel to a take-up reel. The top sprocket, which is free-wheeling, engages the top row of sprocket holes in the film and is rotatably driven by the film while keeping the film taut for scanning of the individual frames.
Because there is no apparatus between the sprocket wheels, it is possible to illuminate the film from behind and sense the light transmitted through the film with a CCD, with none of the slack caused by axially displaced sprocket wheels, and with no interference from intermediate transport mechanisms such as a pull-down claw. Since the film is always maintained taut on the sprockets, registration of the film can be assured.
In an alternate embodiment of the invention, the film is suspended between a driven sprocket wheel and a stationary disc having a smooth track over which the upper margin of the film can glide as it is advanced by the driven wheel.
In order to stabilize the film in the direction of advancement, as well as laterally, the sprockets are provided with a cross sectional profile which facilitates their entry into the film perforations and enables them to tension the film to prevent unwanted movement of the film during each scan of a frame.